Treatment of Gas of Industrial Origin
The nature of the gaseous effluents that can be treated is varied, non-limitative examples thereof are syngas, combustion fumes, refinery gas, Claus tail gases, biomass fermentation gases, cement plant gases and blast furnace gases.
All these gases contain acid compounds such as carbon dioxide (CO2), hydrogen sulfide (H2S), carbon oxysulfide (COS), carbon disulfide (CS2) and mercaptans (RSH), mainly methylmercaptan (CH3SH), ethylmercaptan (CH3CH2SH) and propylmercaptans (CH3CH2CH2SH).
For example, in the case of combustion fumes, CO2 is the acid compound to be removed. In fact, carbon dioxide is one of the greenhouse gases widely produced by human activities and it has a direct impact on atmospheric pollution. In order to reduce the amounts of carbon dioxide discharged to the atmosphere, it is possible to capture the CO2 contained in a gaseous effluent.
Treatment of Natural Gas
In the case of natural gas, three main treating operations are considered: deacidizing, dehydration and stripping. The goal of the first stage, deacidizing, is to remove acid compounds such as carbon dioxide (CO2), as well as hydrogen sulfide (H2S), carbon oxysulfide (COS), carbon disulfide (CS2) and mercaptans (RSH), mainly methylmercaptan (CH3SH), ethylmercaptan (CH3CH2SH) and propylmercaptans (CH3CH2CH2SH). The specifications generally admitted for deacidized gas are 2% CO2, or even 50 ppm CO2, the natural gas being thereafter subjected to liquefaction; 4 ppm H2S and 10 to 50 ppm volume of total sulfur. The dehydration stage then allows to control the water content of the deacidized gas in relation to the transport specifications. Finally, the natural gas stripping stage allows to guarantee the dew point of the hydrocarbons in the natural gas, here again according to transport specifications.
Deacidizing is therefore often carried out first, notably in order to remove the toxic acid gases such as H2S in the first stage of the chain of processes and thus to avoid pollution of the various unit operations by these acid compounds, notably the dehydration section, the condensation and separation section intended for the heavier hydrocarbons.
Acid Compounds Removal by Absorption
Deacidizing gaseous effluents, such as natural gas and combustion fumes for example, as well as syngas, refinery gas, Claus tail gas, biomass fermentation gas, cement plant gas and blast furnace gas, is generally carried out by washing with an absorbent solution. The absorbent solution allows to absorb the acid compounds present in the gaseous effluent (notably H2S, mercaptans, CO2, COS, CS2).
The solvents commonly used today are aqueous solutions of primary, secondary or tertiary alkanolamine, in combination with an optional physical solvent. Document FR-2,820,430, which provides gaseous effluent deacidizing methods, can be mentioned by way of example. U.S. Pat. No. 6,852,144, which describes a method of removing acid compounds from hydrocarbons, can also be mentioned. The method uses a water-methyldiethanolamine or water-triethanolamine absorbent solution containing a high proportion of a compound belonging to the following group: piperazine and/or methylpiperazine and/or morpholine.
For example, in the case of CO2 capture, the absorbed CO2 reacts with the amine present in solution according to a reversible exothermic reaction known to the person skilled in the art and leading to the formation of hydrogen carbonates, carbonates and/or carbamates, allowing removal of the CO2 from the gas to be treated. Similarly, for the removal of H2S from the gas to be treated, the absorbed H2S reacts with the amine present in solution according to a reversible exothermic reaction known to the person skilled in the art and leading to the formation of hydrosulfide.
Another essential aspect of the operations for treating industrial gas or fumes by a solvent is the separation agent regeneration stage. Regeneration through expansion and/or distillation and/or entrainment by a vaporized gas referred to as “stripping gas” is generally provided depending on the absorption type (physical and/or chemical).
One of the main limitations of the solvents commonly used today is the necessity of using high absorbent solution flow rates, which leads to a high energy consumption for solvent regeneration, as well as substantial equipment sizes (columns, pumps, etc.). This is particularly true in cases where the acid gas partial pressure is low. For example, for a 30 wt. % monoethanolamine aqueous solution used for post-combustion CO2 capture in a thermal power plant fume, where the CO2 partial pressure is of the order of 0.1 bar, the regeneration energy represents approximately 3.9 GJ per ton of CO2 captured (reference case, CASTOR project, post-combustion capture pilot unit of the Esbjerg power plant). Such an energy consumption represents a considerable operating cost for the CO2 capture method.
In general terms, for treating acid effluents that comprise acid compounds such as H2S, mercaptans, CO2, COS, SO2, CS2 for example, using amine-based compounds is interesting because of their ease of use in aqueous solution. However, when deacidizing these effluents, the absorbent solution may degrade, either through thermal degradation or through side reaction with the acid gases to be captured, and with other compounds contained in the effluents, such as oxygen, the SOx and the NOx contained in industrial fumes for example. These degradation reactions affect the proper functioning of the method: solvent efficiency decrease, corrosion, foaming, etc. Due to these degradations, it is necessary to carry out solvent purification by distillation and/or ion exchange and to provide make-up amine. By way of example, the make-up amine added in a post-combustion CO2 capture method using a 30 wt. % monoethanolamine absorbent solution represents 1.4 kg amine per ton of CO2 captured, which significantly increases the operating cost of a capture unit.
Finally, these degradation reactions limit the operating conditions of the method, notably the temperature at which solvent regeneration is conducted. By way of example, increasing the regenerator temperature by 10° C. doubles the thermal degradation rate of monoethanolamine. The regeneration of alkanolamine aqueous solutions such as monoethanolamine is therefore carried out at regenerator bottom temperatures of the order of 110° C., or even 130° C. for more stable amines such as methyldiethanolamine. As a result of these regenerator bottom temperatures, the acid gases (H2S, CO2, COS, CS2 etc.) are obtained at moderate pressures ranging from 1 to 3 bars. Depending on the nature of the regenerated acid gas and on applications, the acid gas can be sent to a treating unit or it can be compressed in order to be reinjected and sequestered.
It is difficult to find a stable absorbent compound allowing to remove acid compounds in any effluent type and allowing the deacidizing method to operate at a lesser cost. The applicant has found that N,N,N′,N′-tetramethylhexane-1,6-diamine or TMHDA, alone or in admixture with some wt. % of primary or secondary amines, is of great interest in all the gaseous effluent treatment methods intended for acid compounds removal.
However, most absorbent aqueous solutions comprising N,N,N′,N′-tetramethylhexane-1,6-diamine or TMHDA, alone or in admixture with some wt. % of primary or secondary amines, exhibit a liquid-liquid phase separation upon CO2 absorption under the absorber conditions. In form of two separate phases, the stream of acid compounds transferred from the gas to the absorbent solution would be highly impacted and the column height would have to be adjusted accordingly. This phenomenon therefore poses serious implementation problems and, considering the complexity of the system, it is difficult to model. Surprisingly, the applicant has found that adding some wt. % of particular primary or secondary amines to an aqueous solution of N,N,N′,N′-tetramethylhexane-1,6-diamine allows to obtain a single-phase absorbent solution under the conditions of absorption of acid gases such as CO2.